1. Field of the Invention
The present invention relates to a semiconductor device in which an analog circuit and a digital circuit exist and more particularly, to a semiconductor device to protect an analog circuit from a noise generated from a digital circuit.
2. Description of the Background Art
As a semiconductor device such as an LSI becomes highly integrated and multifunctional recently, an analog circuit and a digital circuit are formed on the same semiconductor substrate, the same chip, for example. An electric signal generally used in the digital circuit has a large amount of signal variation and has a high frequency. As a result, when this affects the analog circuit through the same semiconductor substrate as a noise, the problem is that the precision of the analog circuit is likely to be lowered.
In order to protect the analog circuit from such noise, a semiconductor device in which a deep well is formed in a region of the analog circuit is proposed. Alternatively, as disclosed in Alan Hastings, “The Art of ANALOG LAYOUT”, Second Edition, Person Prentice Hall, 2006, p. 173, a semiconductor device in which a substrate potential fixing region is formed between a digital circuit and an analog circuit is proposed. Thus, the analog circuit is protected from the noise by separating the digital circuit and the analog circuit.
However, when a semiconductor element in which a potential of its well cannot be fixed such as a parasitic bipolar, a well resistor, and a varactor, having an N type well as its terminal is formed in the analog circuit, the problem is that the semiconductor element is not sufficiently protected from the noise generated from the digital circuit and the precision of the analog circuit is lowered.